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Infrared Nanospectroscopy (AFM-IR)
Infrared microscopy technique / From Wikipedia, the free encyclopedia
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AFM-IR (atomic force microscope-infrared spectroscopy) or infrared nanospectroscopy is one of a family of techniques[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] that are derived from a combination of two parent instrumental techniques. AFM-IR combines the chemical analysis power of infrared spectroscopy and the high-spatial resolution of scanning probe microscopy (SPM). The term was first used to denote a method that combined a tuneable free electron laser with an atomic force microscope (AFM, a type of SPM) equipped with a sharp probe that measured the local absorption of infrared light by a sample with nanoscale spatial resolution.[16][17][18]
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Originally the technique required the sample to be deposited on an infrared-transparent prism and be less than 1μm thick. This early setup improved the spatial resolution and sensitivity of photothermal AFM-based techniques from microns[7] to circa 100 nm.[8][9][10][16][19][20] Then, the use of modern pulsed optical parametric oscillators and quantum cascade lasers, in combination with top-illumination, have enabled to investigate samples on any substrate and with increase sensitivity and spatial resolution. As most recent advances, AFM-IR has been proved capable to acquire chemical maps and nanoscale resolved spectra at the single-molecule scale from macromolecular self-assemblies and biomolecules with circa 10 nm diameter,[18][17][21][22] as well as to overcome limitations of IR spectroscopy and measure in aqueous liquid environments.[23]
Recording the amount of infrared absorption as a function of wavelength or wavenumber, AFM-IR creates an infrared absorption spectra that can be used to chemically characterize and even identify unknown samples.[12][15][24] Recording the infrared absorption as a function of position can be used to create chemical composition maps that show the spatial distribution of different chemical components. Novel extensions of the original AFM-IR technique[18][17] and earlier techniques[1][2][3][4][6][7][24] have enabled the development of bench-top devices capable of nanometer spatial resolution, that do not require a prism and can work with thicker samples, and thereby greatly improving ease of use and expanding the range of samples that can be analysed. AFM-IR has achieved lateral spatial resolutions of ca. 10 nm, with a sensitivity down to the scale of molecular monolayer[25] and single protein molecules with molecular weight down to 400-600 kDa.[18][17]
AFM-IR is related to techniques such as tip-enhanced Raman spectroscopy (TERS), scanning near-field optical microscopy (SNOM),[26] nano-FTIR and other methods of vibrational analysis with scanning probe microscopy.